1. Field of the Invention
Embodiments of the present invention generally relate to a method of forming a dielectric. More particularly, embodiments of the invention relate to a method of forming a silicon oxynitride (SiOxNy) dielectric.
2. Description of the Related Art
As integrated circuit sizes and the sizes of the transistors thereon decrease, the drive current required to increase the speed of the transistor has increased. The drive current increases as the capacitance increases, and capacitance=kA/d, wherein k is the dielectric constant, d is the dielectric thickness, and A is the area of the device. Decreasing the dielectric thickness and increasing the dielectric constant of the gate dielectric are methods of increasing the gate capacitance and the drive current.
Attempts have been made to reduce the thickness of dielectrics, such as silicon dioxide (SiO2) dielectrics, below 20 Å. However, the use of SiO2 dielectrics with thicknesses below 20 Å often results in undesirable performance and durability. For example, boron from a boron doped electrode can penetrate through a thin SiO2 dielectric into the underlying silicon substrate. Also, there is typically an increase in gate leakage current, i.e., tunneling current, with thin dielectrics that increases the amount of power consumed by the gate. Thin SiO2 gate dielectrics may be susceptible to negative-channel metal-oxide semiconductor (NMOS) hot carrier degradation, in which high energy carriers traveling across the dielectric can damage or destroy the channel. Thin SiO2 gate dielectrics may also be susceptible to positive channel metal oxide semiconductor (PMOS) negative bias temperature instability (NBTI), wherein the threshold voltage or drive current drifts with operation of the gate.
A method of forming a dielectric layer suitable for use as the gate dielectric layer in a MOSFET (metal oxide semiconductor field effect transistor) includes nitriding a thin silicon oxide film in a nitrogen-containing plasma. Increasing the net nitrogen content in the gate oxide to increase the dielectric constant is desirable for several reasons. For example, the bulk of the oxide dielectric may be lightly incorporated with nitrogen during the plasma nitridation process, which reduces the equivalent oxide thickness (EOT) over the starting oxide. This may result in a gate leakage reduction, due to tunneling during the operation of a field effect transistor, at the same EOT as the oxide dielectric that is not nitrided. At the same time, increased nitrogen content may also reduce damage induced by Fowler-Nordheim (F-N) tunneling currents during additional processing operations, provided that the thickness of the dielectric is in the F-N range. Another benefit of increasing the net nitrogen content of the gate oxide is that the nitrided gate dielectric is more resistant to the problem of gate etch undercut, which in turn reduces defect states and current leakage at the gate edge.
In U.S. Pat. No. 6,610,615, titled “Plasma Nitridation for Reduced Leakage Gate Dielectric Layers” and issued on Aug. 26, 2003, McFadden, et al. compares nitrogen profiles in a silicon oxide film for both thermal and plasma nitridation process. The nitrided oxide films are disposed on a silicon substrate. Testing of the thermal nitrided oxide films nitrogen profiles in the crystalline silicon beneath the oxide film shows a first concentration of nitrogen at a top surface of an oxide layer, a generally declining concentration of nitrogen deeper in the oxide, an interfacial accumulation of nitrogen at the oxide-silicon interface, and finally, a nitrogen concentration gradient that is generally declining with distance into the substrate. In contrast, it can be shown that the plasma nitridation process produces a nitrogen profile that is essentially monotonically decreasing from the top surface of the oxide layer through the oxide-silicon interface and into the substrate. The undesirable interfacial accumulation of nitrogen observed with a thermal nitridation process does not occur with the ionic bombardment of the nitrogen plasma. Furthermore, the nitrogen concentration in the substrate is lower, at all depths, than is achieved with the thermal nitridation process.
A benefit of increasing nitrogen concentration at the gate electrode-gate oxide interface is that dopant diffusion with dopants, such as boron, from polysilicon gate electrodes into or through the gate oxide is reduced. This improves device reliability by reducing defects in the bulk of the gate oxide caused by, for example, in-diffused boron from a boron doped polysilicon gate electrode. Another benefit of reducing nitrogen content at the gate oxide-silicon channel interface is the reduction of fixed charge and interface state density. This improves channel mobility and transconductance.
A nitrogen containing silicon oxide dielectric material that can be used with a physical thickness that is effective to reduce current leakage density and provide high gate capacitance is needed. The nitrogen containing silicon oxide dielectric material must have a dielectric constant that is higher than that of silicon dioxide. Typically, the thickness of such a dielectric material layer is expressed in terms of the equivalent oxide thickness (EOT). Thus, the EOT of a dielectric layer is the thickness that the dielectric layer would have if its dielectric constant were that of silicon dioxide.
A SiOxNy dielectric can be formed by incorporating nitrogen into a SiO2 layer or forming a silicon nitride layer on a silicon substrate and incorporating oxygen into the layer by an oxidation process involving oxygen or precursor gases that contain nitrogen and oxygen.
However, as device geometries continue to shrink, there remains a need for an improved method of depositing silicon oxynitride dielectrics that have lower EOT than conventionally deposited silicon oxynitride films.